XI PREDATION ON MUSSELS IN CULTURE BY SILVER BREAM, SARBA

K. K. APPUKUTTAN

Rhabdosargus sarba shoals, as observed in Vizhinjam area during 1979, play havoc to the cultured mussels. 250 ropes containing good quantity of harvestable mussels are reported to have been comi^etely destroyed by predation by this . Attempts to control predation did not yield results. In addition to lobster predation noticed comnHmly in the area, Rhabdosargus sarba has added a new dimension to the problem.

INTRODUCTION their stomach. All the three specimens of Arius sp, had stomach full of mussels and out of 5 specimens of Davies (1970, Proc. Symp. on Mollosca, 3: 873-884, Arothron nigroptmctata, two had mussels. However, M.B.A. ladia^), while dealing with mussel as world samples of collected in subsequent months failed food resource has discussed various problems in culture tQ reveal mussels in the stomach contents. The damage and attached great significance to the predation caused caused to the seed on the ropes was observed to be very by in the farm. Important among them are negilible. In 1977 and 1978 also only the three crabs, fish, and also flat-fishes from European waters. mentioned above caused minor damage to the seed. He has silso mentioned about damages caused by diving ducks and oyster catchers preying on intertidal During January 1979, 250 seeded ropes released in mussels and pointed out that in the development of the floating rafts inside the bay, were attacked by a mussel culture in new areas, completely different pre­ shaol of silver bream, Rhabdosargus sarba. Eflforts to dation problems may arise, for example midwater catch the fishes by hook and line did not succeed. predation, and concludes that raft culture is not a sure However fish traps kept in the bay enabled collection of remedy for predation. fishes ranging from 360 to 470 mm in total length during the first week of February. Examination of the stomach OBSERVATIONS contents revealed stomach full of crushed mussels, clearly indicating that these fishes were heavily feeding Experiments on floating raft culture at Vizhinjam Bay on mussels on the rope. By the end of February, the from 1976 showed that predation by fishes was not a entire stock of seeded mussels was eaten up by the serious problem till 1979. While examining the seeded shoals of fish (Plate I, a). Control of predation by ropes in 1976 browsing of mussels by fisheswa s noticed. gill netting was tried but it was not efiective. It was Fish traps were kept in the rafts to collect these fishes also understood that 1500 seeded ropes kept in the in the vicinity of the raft area. Diodon hystrix, Arius floating rafts of the Pilot Project on mussel culture of sp., Arothron nigropmctatus, Ostracion spp., Chaetodon Kerala State Fisheries Department were also reported spp., and Acanthurus sp., were collected and the stomach to have been completely destroyed by the shoals of the contents of these fishes were examined. It was found same species, Rhadosargus sarba by the end of April. that the firstthre e species of fishesfee d on young mussels of 20 to 30 mm size. Out of 16 numbers of Diodon Full grown mussels on ropes kept suspended from hystrix collected during October (ranging from 175- rafts in the Bay were also not spared from predation. 295 mm in total length) 8 specimens had mussels in The fishes crushed the shells and fed upon the flesh

44 MUSSEL FARMINO 6

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PLATE I. a. Damaged mussel ropes, b. Crushed shells of full grown mussels, c. Rhabdosargussarba. leaving remnants of empty shell still attached to ropes cages as bait, for luring lobsters into the trap. It is (Plate I, b). understood from fishermen that mussels are considered as the best bait for lobster fishing. This Silver bream, locally called 'Thavanupara' (family: ) are carnivorous, inhabiting shallow Mahadevan (Persomd communication) has stated that waters. The body is silvery with golden longitudinal lines unusually large shoals Q{ Drepane punctata appear in the along scale rows. A yellow streak across belly is present. pearl banks during peari fishery times attracted by the (Plate I, c). In the upper and lower jaw, rows of rotting flesh of the pearl oysters at the bottom, discarded compressed incissors and rounded molars are present. earlier by fisher-folk cutting open oysters while tiie Posterior molar in each jaw greatly enlarged. T^e teeth fishing is in progress. Similarly the probable reason structure is well adapted for crunching hard objects. for the entey of shoak of ^^r bream in the aoea can .be attributed to the dynamitiiig for fishing in this area. REMARKS During January and February the local fidxermoiwer e dynamiting uound the breakwater area for mackerel It may be of interest to mention here that this fish and caranx shoal. There is every possibility of the does not form significant fishery in this area. The only silvn: bream shoal being distufbed from the natural fish so far been reported as causing damage to mussd habitat and aoridentally entering the mussel culture in natural bed is flat fishes (Davies, 1970^) in European area. As the fi^berm^a could collect only a fraction of waters. The present report shows that silver bream the fishes by dynamUing, the rest get battered attract­ may pose serious problems for mussel culture at ing pugnacious feeding by the silver bream. Vi2^njam area. It may not be out of place to mention that during experiments on mussel culture inside the Several methods h«ve been developed in foreign bay it was observed that k^ters also cause damage to countries to solve theinroblem of large^Ue predatory cultured mussels on a minor scale. Lobster cages were {^acks. Hanson (1974, Open Sea Markulture, kept in the farm and found that during November to Dowden, Hutchinson & Ross. Inc. 410 pp.^) lists, air January period number of lobsters could be collected barriers, electrical barriers, acoustical barriers, chemical from the farm area. In the natural bed around controls and barriers amongst them. It has Vizhinjam also mussel predation by lobsters was ob­ to be seen whether the present instance of predation by served. They cut the live shells and eat away the flesh Khabdosargus is an isolated event or is a perennial leavmg the broken shells attached to substratum. It nuisance in the area before we can think of suggesting may be of interest to note that the lobster fishermen measures, «^n the frame-work of practicaWlity to keep mussels collected from the natural bed inside the overcome the attacks.

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